Ahmed Elmohamady's late header snatched three points to send Hull second in the Championship and deny stubborn Derby.
After a poor first-half display, Hull took the lead shortly after the break when substitute Gedo headed home with just his second touch of the ball.
Jamie Ward punished goalkeeper David Stockdale's mistake to put the visitors level with six minutes remaining.
But Elmohamady fired in from a corner before Derby's Gareth Roberts was sent off for a lunge on Nick Proschwitz.
It was a miserable end to an evening that began promisingly for a Derby side that enjoyed the better of the early exchanges.
They could not make their pressure count through as Conor Sammon poked wide and a last-ditch tackle by Jack Hobbs barred Ward's way to goal.
City sparked into life as the half ended with Stephen Quinn's near-post header skimming just wide of Adam Legzdins' right-hand post.
Egypt international Gedo's header was more precise as he nodded Robbie Brady's delivery back across the Derby goalkeeper for the opener.
The Egyptian came close to a second with a volley just over, but Derby pressed forward and Sammon thumped the underside of the bar before almost inadvertently turning in Ward's effort.
Ward's industry was rewarded when Stockdale spilled Craig Bryson's cross to gift him a simple tap-in.
But Derby were again undone in the air as Elmohamady converted from another Brady delivery before Roberts was dismissed and Proschwitz picked himself up to miss a fine chance to make the game safe.
VIEW FROM THE DRESSING ROOM
Hull manager Steve Bruce:
"The boy Gedo showed his class. He's an international footballer and I think he's got 19 goals in 30 games so we know he has got quality.
"A big credit goes to the chairman. He's worked wonders to get him here.
"I didn't think it would be possible to get two international footballers on loan, and Ahmed Fathi will be a big player for us too."
Derby manager Nigel Clough:
"It would have been a thoroughly deserved point. It was a very good away performance at one of the favourites for promotion and I can't ask any more from our players whatsoever.
"That's two major decisions in four days that have gone against us.
"We don't usually appeal these things unless it's blatantly obvious, so I don't think we will be, but it certainly wasn't intentional."
Hull City 34 Stockdale 05 Chester 06 Hobbs 15 McShane 08 Evans Booked (Rosenior - 90' ) 10 Koren (Mohamed Gedo - 46' ) 27 Elmohamady Booked 29 Quinn 31 Meyler Booked 11 Simpson (Proschwitz - 46' ) 30 Brady Substitutes 01 Jakupovic 02 Rosenior 17 Fathi 14 Cairney 16 Olofinjana 22 Mohamed Gedo 33 Proschwitz Derby County 13 Legzdins 02 Brayford 03 Roberts Dismissed 06 Keogh 23 Freeman (Robinson - 80' ) 04 Bryson Booked 07 Coutts Booked (Buxton - 85' ) 08 Hendrick 19 Hughes 10 Ward 32 Sammon (Jacobs - 73' Booked ) Substitutes 01 Fielding 05 Buxton 21 Gjokaj 22 Hoganson 12 Jacobs 18 Davies 11 Robinson
Ref: Robert Madley